In the field of mobile communication devices typified by smartphones, a drastic increase in communication traffic and multi-functionality have recently progressed. In order to meet the increase in communication traffic, the number of bands has been increased, while the mobile communication devices are required to have various functions without enlarging the shape of them. Various parts to be used for these mobile communication devices must therefore be smaller and have higher performance.
An oxide single crystal such as lithium tantalate (LT) and lithium niobate (LN) is a typical piezoelectric material and has been used widely as a material of surface acoustic wave (SAW) devices. The oxide single crystal used as a piezoelectric material enables band broadening because an electromechanical coupling factor, which indicates the conversion efficiency of electromagnetic energy into dynamic energy, is large. However, it has low stability against a temperature change, and the frequency to which it can respond varies with the temperature change. The low stability against the temperature change owes to the thermal expansion coefficient of the oxide single crystal.
For improving the temperature stability in the case where the oxide single crystal is used as a piezoelectric material, there is provided, for example, a method comprising steps of: laminating, with an oxide single-crystal wafer, a material having a thermal expansion coefficient smaller than that of the oxide single crystal, more specifically, a sapphire wafer; and thinning (e.g. grinding) the oxide single-crystal wafer side of the resulting laminate to a thickness of from several to tens of μm to suppress the influence of thermal expansion of the oxide single crystal (Non-Patent Document 1). In this method, however, the oxide single-crystal wafer is ground after lamination, so that a large portion of the oxide single-crystal wafer is wasted. Thus, it is inferior in terms of efficient use of the material. In addition, lithium tantalate or lithium niobate used as the oxide single crystal is an expensive material so that there is a demand for a method involving highly efficient use of the material and being capable of reducing the waste so as to reduce a production cost.
As an example of the method of producing a SOI wafer, the Small-Cut method, in short, comprises steps of: laminating a silicon wafer having a hydrogen ion-implanted layer with a support wafer, and heat-treating the resulting laminate around 500° C. to thermally split the laminate along the ion-implanted layer (Patent Document 1). In order to enhance the efficient use of an oxide single-crystal wafer, an attempt has been made to use the oxide single-crystal wafer instead of the silicon wafer used in the Small-Cut method to form an oxide single-crystal film on the support wafer (Non-Patent Documents 2 and 3).
Non-Patent Document 2 reports a method of producing a LTMOI (lithium-tantalate-metal-on-insulator) structure comprising steps of: forming a 121-nm thick Cr metal layer on a surface of a lithium tantalate wafer having an ion-implanted layer; laminating the wafer with a SiO2 substrate having a thickness of hundreds of nm, while keeping the metal layer therebetween; heat-treating the resulting laminate at a temperature of from 200 to 500° C. to split the laminate along the ion-implanted layer, thereby transferring a lithium tantalate film onto the SiO2 substrate via the metal layer; and laminating the lithium tantalate wafer with the surface of the SiO2 substrate on the side opposite to the surface to which the lithium tantalate film has been transferred. Non-Patent Document 3 reports a method of thermally transferring a lithium tantalate film onto the silicon wafer comprising steps of: laminating a silicon wafer with a lithium tantalate wafer having an ion-implanted layer; and heat-treating the resulting laminate at 200° C. to split the laminate along the ion-implanted layer.